1) Laboratoire De Cytogen6tique, Place Croix du Sud 5, B-1348 Louvain-la-Neuve
2) Unitd De Phytopathologie, Place Croix du Sud 2, B-1348 Louvain-la-Neuve, Belgium
Pseudomonas fuscovaginae (Miyajima, Tanii and Akita 1983) is the causal agent of brown sheath rot of rice in the tropical upland fields. The disease affects panicle exertion causing partial or total sterility.
The bacteria produce in culture and probably in vivo low molecular-weight compounds, causing the known symptoms of the disease in the field. The major bioactive compound of this toxin is syringotoxin (Batoko et al. 1994a), one of the non-host specific toxins of the lipodepsinonapeptide family, considered as virulence factors of different pathovars of Pseudomonas syringas (Gross 1991).
Our studies on the role of this toxin in pathogenesis showed that the severity of damage caused by this compound on the whole plant in greenhouse, seedling in vitro, and rice cells in culture, differs according to genotype.
In greenhouse for example, we used Facagro 57 (H57), an Indica hybrid [IR 23412-RR-19-3-B (IRRI) X B 2980 b-SR-26232 (Indonesia)] from Burundi exhibiting a high susceptibility to the disease, and Ambalalava (MI283), a rustic indica variety from Malagasy exhibiting field tolerance to the disease. Plants of these two varieties were infiltrated at the booting stage with the purified toxin. Inhibition of elongation of the peduncle and internodes are the best quantitative symptoms to differentiate susceptibility among varieties to the disease (Detry et al. 1991). Evaluation of this parameter after toxin treatment showed that the tolerant genotype (MI283) was less affected than the susceptible one.
The toxin enhanced seed germination, but induced a drastic inhibition of seedling elongation which was correlated to varietal susceptibility to the disease in the field (Batoko et al. 1994a). After denaturation of the bioactive compounds of the extract, all previously observed effects were lost.
Treating rice callus with the toxin resulted in a high electrolyte leakage. The kinetics of leaching and the percentage of total electrolyte lost showed that the toxin per se affected the cells of red tolerant genotype less dramatically than those of susceptible ones (Batoko et al. 1994b). In fact, phytotoxic lipodepsinonapetides affect fungal and plant cells partially by a detergent-like action and inhibition of plasma membrane H+-ATPase (Che et al. 1992), inducing electrolyte leakage (lacobellis et al. 1992).
We have selected insensitive cells to the toxin by in vitro technique, and plants regenerated from these cells and their offspring were evaluated both in greenhouse and in field conditions for their susceptibility to the disease.
These findings show that the susceptibility of rice genotype to Pseudomonas fuscovaginae disease is correlated with its sensitivity to the bacterial toxin, which is non-host specific.
References
Batoko, H., J. Bouharmont and H. Maraite, 1994a. Inhibition of rice (Oryza sativa L.) seedling elongation by a Pseudomonas fuscovaginae toxin. Euphytica. (in press)
Batoko, H., D. Hsissou, J. Bouharmont, H. Maraite and J-M. Kinet, 1994b. Phytotoxicity of a Pseudomonas fuscovaginae toxin on rice cells: enhancement of electrolyte leakage from treated calli. Archives Internationales de Physiologic, de biochimie et de biophysique 102: 1-11.
Che, F-.S., K. Kasama, N. Fukuchi, A. Isogai and A. Suzuki, 1992. Bacterial phytotoxins, syringomycin, syringostatin and syringotoxin, exert their effect on the plasma membrane H+-ATPase partly by a detergent-like action and partly by inhibition of the enzyme. Physiol. Plantarum 86: 518-524.
Detry, J.F., E. Duveiller and H. Maraite, 1991. Mise au point d`une methode d'evaluation de la resistance de cultivars de riz a Pseudomonas fuscovaginae. Parasitica 47: 151-163.
Gross, D. C., 1991. Molecular and genetic analysis of toxin production by pathovars of Pseudomonas syringas. Ann. Rev. Phytopath. 29, 247-278.
Iacobellis, N. S., P. Lavernicocca, I. Grourina, M. Simmaco and A. Ballio, 1992. Phytotoxic properties of Pseudomonas syringas pv. syringas toxins. Physiol. Mol. Plant Pathol. 40: 107-116.
This work was supported by EEC Grant No. TS-3-CT92-0095 and a fellowship grant to H. Batoko.